1. Field Of The Invention
This invention relates to radio broadcasting and, more particularly, to methods of and apparatus for providing a service channel for an amplitude modulated compatible digital broadcast signal.
2. Description Of Related Art
There has been increasing interest in the possibility of broadcasting digitally encoded audio signals to provide improved audio fidelity. Several approaches have been suggested. One such approach, set forth in copending patent application Ser. No. 08/206,368, filed Mar. 7, 1994, now U.S. Pat. No. 5,588,022 assigned to the assignee hereof, teaches a method for simultaneously broadcasting analog and digital signals in a standard AM broadcasting channel. An amplitude modulated radio frequency signal having a first frequency spectrum is broadcast. The amplitude modulated radio frequency signal includes a first carrier modulated by an analog program signal. Simultaneously, a plurality of digitally modulated carrier signals are broadcast within a bandwidth which encompasses the first frequency spectrum. Each of the digitally modulated carrier signals is modulated by a portion of a digital program signal. A first group of the digitally modulated carrier signals lies within the first frequency spectrum and is modulated in quadrature with the first carrier signal. Second and third groups of the digitally modulated carrier signals lie outside of the first frequency spectrum and are modulated both in-phase and in-quadrature with the first carrier signal. Both transmitters and receivers are provided in accordance with that method.
The waveform in the AM compatible digital audio broadcasting system described in U.S. Pat. No. 5,588,022, hereby incorporated herein by reference, has been formulated to provide optimal data throughput for the digital signal while avoiding cross talk into the analog AM channel. Multiple carriers are employed by means of orthogonal frequency division multiplexing (OFDM) to bear the communicated information. If additional information besides broadcaster program material is programmed concurrently with the program material, data services can be used over the commercial broadcasting channel to communicate any type of information that has been coded in a binary format.
There has been extensive work performed in the past to provide systems for transmitting data simultaneously with audio. For example, U.S. Pat. No. 4,379,947 discloses an FM broadcasting system for transmitting a data signal simultaneously with an auxiliary audio program on the same subcarriers of the FM transmission channel. The main program is transmitted on the FM transmission channel carrier and the auxiliary audio program, which may be background music, is transmitted on the subcarriers in a conventional manner. The data signal, which is added to the auxiliary audio program, is a binary digital information signal. A receiver adapted for use with this broadcasting system is capable of receiving one or more of the main program, the auxiliary radio program, and the data signal.
U.S. Pat. No. 4,512,013 discloses a technique for transmitting an entire analog speech signal and a modulated data signal over a transmission channel such as a common analog telephone speech channel. U.S. Pat. No. 5,146,612 discloses a technique for using a subcarrier frequency of a radio station to transmit, receive, and display a message together with audio reproduction of the radio program.
U.S. Pat. No. 5,191,576 discloses a method for broadcasting digital data for use by mobile receivers traveling in an urban environment in the presence of stray signals and jamming.
U.S. Pat. No. 3,160,812 discloses a composite transmission system utilizing phase shift and amplitude modulation.
U.S. Pat. No. 3,714,375 discloses a secondary phase modulated communication service using AM broadcast stations.
The recently discovered ability to broadcast digitally encoded audio signals in a standard AM broadcasting channel has made possible the ability to provide data service in addition to broadcaster program material using AM digital audio broadcasting systems. Accordingly, there is a need for a system which provides a data service channel concurrent with a commercial broadcasting channel.